(a) Field of the Invention
This invention relates to an improvement in the synthesis of urea. More particularly, it relates to a process for synthesizing urea with an improved thermal economy.
(b) Description of the Prior Art
With the recent steep rise in energy prices, it is now general practice to try to thoroughly recover thermal energy even in the production of urea. The thermal energy therein recovered is reutilized in the urea synthesis system and any extra thermal energy is estimated to be low pressure steam. According to the recent trend, the quantity of steam required for a urea synthesis process is determined by the difference between high pressure steam required in the reaction system for the decomposition and separation of unreacted ammonium carbamate and any excessive ammonia and the low pressure steam recovered (these ammonium carbamate and ammonia will hereinafter be referred to as "unreacted materials"). As the above-defined difference becomes smaller, a urea synthesis process is considered to be improved.
In the above sense, the so-called stripping process has been known as one of the outstanding urea synthesis processes. In the stripping process, urea synthesis effluent containing unreacted materials is subjected to a stripping step by carbon dioxide or ammonia, which is supplied as a raw material, at a pressure substantially equal to the urea synthesis pressure while being heated by high pressure steam of 25 kg/cm.sup.2 G, thereby to decompose and separate same from the resultant urea solution. A resultant gaseous mixture consisting of the thus-separated ammonia and carbon dioxide as well as the carbon dioxide or ammonia, which was used for the decomposition and separation of the unreacted materials, is then condensed under a pressure substantially equal to the stripping pressure. The condensation heat is recovered as low pressure steam of 2-5 kg/cm.sup.2 G. The resulting quantity of steam is so great that some steam would still remain even if consumed in steps capable of using such low pressure steam within the urea synthesis system, such as the condensation step. On the other hand, the urea synthesis effluent from which unreacted materials have been removed by stripping with carbon dioxide as described above is thereafter subjected to a low pressure decomposition under 1-5 kg/cm.sup. 2 G and substantially all the remaining unreacted materials are decomposed in and separated from the urea synthesis effluent. The urea synthesis effluent from which unreacted materials have been removed through stripping by ammonia still contains a large quantity of ammonia therein. Thus, such urea synthesis effluent is first subjected to a high pressure decomposition under 10-25 kg/cm.sup.2 G and then to a low pressure decomposition under 1-5 kg/cm.sup.2 G. As heat sources for such high and low pressure decompositions, there are employed high pressure steam and low pressure steam. Generally speaking, the above-described stripping process is capable of recovering a considerable quantity of low pressure steam, but it requires a large quantity of high pressure steam for the following reasons. Namely, the stripping operation in the stripping process becomes easier to carry out as its pressure lowers. However, a urea synthesis effluent obtained by performing a urea synthesis at a relatively lower pressure contains a considerable amount of unreacted materials since the conversion ratio remains low at such a low pressure. Accordingly, it is necessary to use a large quantity of high pressure steam for the decomposition and separation of the unreacted materials.
To avoid the above-described difficulties, it becomes necessary to conduct a urea synthesis process at a higher urea synthesis pressure and temperature while maintaining ammonia in an excessive molar ratio with respect to carbon dioxide. This, however, creates another problem, namely, a higher urea synthesis pressure requires the stripping operation to be performed at a high pressure. It is, of course, possible to raise the operation temperature of the stripping operation with a view toward rendering the stripping operation easier. However, a stripping operation at high temperatures is accompanied by the drawback that more resultant urea is hydrolyzed.